1. Field of the Invention
The present invention relates to an inkjet printing apparatus and a driving control method which enable registration adjustment for preventing a relative misregistration between droplet landing points of printing elements of a printhead.
2. Description of the Related Art
There are various printing apparatuses, including printing means provided in printers, copying machines, and facsimile machines, for printing images and other objects and printout devices used with multifunctional electronic apparatuses such as computers and word processors or workstations. These printing apparatuses are designed to print images and other objects on printing media such as paper and plastic film in accordance with image information.
Such printing apparatuses can be classified by printing method as inkjet, wire dot-matrix, thermal, laser-beam and other printing apparatuses.
Among these printing apparatuses, inkjet printing apparatuses discharge ink drops through a printhead onto a printing medium to print. Compared with other types of printing apparatuses, the inkjet printing apparatuses have a number of advantages. For example, inkjet printing apparatuses can be easily designed to print in high-definition and are faster and quieter, and lower in cost.
Further, color outputs such as color pictures have grown in importance in recent years and many color inkjet printing apparatuses that print high-quality images comparable to silver-based photographic prints have been developed.
Such an inkjet printing apparatus typically uses a printhead on which multiple printing elements are arranged and multiple ink nozzles and ink channels are integrated in order to increase printing speed, and has multiple such printing heads in order to support color printing.
While various printing technologies for printers are known, attention is being given to inkjet printing technology today for reasons such as the capability to print on printing media such as paper in a non-contact manner, ease of color printing, and quietness.
Serial printing technology is commonly used in inkjet printers because of low cost and ease of downsizing, among other reasons. In the serial printing technology, a printhead that discharges ink in accordance with desired print information is attached and is driven to scan forward and backward in the direction perpendicular to the direction in which printing media are fed.
These inkjet printers have been significantly sophisticated recently and high printing speeds comparable to laser-beam printers have been achieved. Furthermore, demand for faster color image printing is growing with increase in processing speed of personal computers and proliferation of the Internet.
To achieve high-image-quality printing, registration adjustment is required which prevents relative misregistrations between landing points of ink droplets from nozzles of a printhead.
There are many techniques for registration adjustment, including a method of preventing misregistration between droplet landing points of color nozzles and a method of preventing misregistration between landing points of droplets of the same color ink in first (forward) and second (backward) scan directions in bidirectional printing. Such methods are implemented in many products as known techniques.
FIG. 1 shows an exemplary arrangement of nozzles of a printhead 101. The printhead shown in FIG. 1 has multiple pairs of nozzle arrays to enable discharge of different inks. A nozzle array 102 consisting of even-numbered nozzles 104, each having an even number assigned to it for convenience, is located to the left of an ink supply path 106. A nozzle array 103 consisting of odd-numbered nozzles 104, each having an odd number assigned to it for convenience, is located to the right of the ink supply path 106. Ink is supplied to the nozzles 104 individually through each individual ink channel 105.
The positional relation between the nozzles 104 is as follows. Two arrays of many nozzles arranged at a pitch py in the y-direction are provided. The two arrays are offset from each other in the x-direction by a distance px equivalent to a predetermined number of pixels. The even-numbered nozzle array 102 and the odd-numbered nozzle array 103 are shifted from each other in the y-direction by a distance of (py/2).
With this arrangement, printing can be performed with a resolution twice as high as the density (resolution) of nozzles per array by adjusting discharge timing between both nozzle arrays. However, registration of landing points between rasters of ink of the same color and registration of landing points between ink discharged from the even-numbered nozzle array 102 and ink discharged from the odd-numbered nozzle array 103 must be adjusted.
A method for adjusting registration is proposed in Japanese Patent Laid-Open No. 2001-129985, for example.
A printhead driving method is commonly used in which multiple nozzles arranged in one line in the column direction (in the y-direction) are divided into groups of nozzles and the printing elements of the nozzle groups are individually driven at different timings (time-divisional driving). The method is described in detail in Japanese Patent Laid-Open No. 2000-071433. By time-divisional driving of printing elements, the ink supply rate and stability can be improved and consumption of power required for discharging can be reduced. Also disclosed is a configuration in which nozzles disposed at regular intervals are grouped into the same block and an order in which blocks are driven is chosen so that adjacent nozzles are not successively driven, thereby reducing the impact of driving of an adjacent nozzle.
Registration can be adjusted by shifting a column of print data by a distance ranging from a half pixel to a number of pixels or by shifting print timing by a predetermined amount of time or by other methods.
The method of shifting a column of print data by a distance ranging from a half pixel to a number of pixels is used in order to roughly adjust registration between landing points of droplets of ink of different colors discharged from nozzles or registration between landing points of droplets of ink of the same color discharged in first and second scan directions in bidirectional printing.
As shown in FIG. 2, if printing is performed with a print resolution of 1200 dpi in the scanning direction of the printhead, a column of print data of 1200 dpi can be shifted by shifting the print data by one or more pixels. Also, by shifting a column of print data by a half pixel, the print data can be shifted by a pixel pitch equivalent to ½ of print resolution. In the example in FIG. 2, 2400 dpi data can be shifted as a unit.
In the method of shifting print timing by a predetermined amount of time, timing of printing is shifted within an amount of time allocated to a column for printing with a predetermined print resolution (column timing). With this method, print timing can be shifted on a cycle-by-cycle basis of a base clock that operates the printing apparatus. This method is used for correcting a small misalignment caused by a difference between individual heads that arose in manufacturing or a difference in printing environment.
However, these methods cannot adjust registration between landing points of nozzles in the same array because they shift landing points by moving nozzle arrays to shift the print starting position.
Misregistration between landing points of ink droplets from the same nozzle array has not posed a significant problem in conventional printheads because the size of a droplet of ink is relatively large, in the range between 5 and 30 pl (picoliters). Accordingly, it is sufficient if registration between landing points can be adjusted at the level of nozzle array. Recently, however, the sizes of ink droplets have been minimized in order to achieve high-quality printing comparable to silver-based photographic prints. Ink droplets as small as 1 to 2 pl can be discharged.
When the size of a droplet is reduced to ½, the number of dots to be placed for printing in the same print area doubles in both vertical and horizontal directions as shown in FIG. 3, that is, four times as many as the number of dots will be required in total. Accordingly, the printing speed will significantly decrease, of course, if the number of nozzles of a printhead, the density of nozzles in an array, and the discharge frequency are the same.
To achieve a faster printing speed than before by using a printhead that discharges such small droplets, a method for increasing the number of nozzles and the density of nozzles arranged in a printhead to increase the coverage area that can be printed at a time or a method for increasing the frequency of discharge of ink droplets must be developed.
During development aimed at reducing droplet size and increasing printing speed, a new kind of problem has arisen associated with such smaller droplet sizes. In particular, the direction in which ink droplets are discharged from a printhead on a carriage that moves quickly in an existing printer system is significantly changed by its air resistance.
The change of the discharge direction changes the landing points of ink droplets both in the scanning direction of the printhead and in the direction in which nozzles are arranged, which of course results in degradation of image quality. Moreover, it has been shown that if the time-divisional driving stated above is performed, misregistration occurs between the landing points of ink droplets discharged from nozzles in the first driven block and the landing points of ink droplets discharged from nozzles in the last driven block. Therefore, particularly misregistration of landing points in the same nozzle array in the scanning direction of the printhead increases because misregistration of landing points caused by the time-divisional driving is combined with misregistration of landing points caused by the air resistance.
As an example of development aimed at reduction of droplet size and increase of printing speed mentioned above, changes to the configuration of printheads are being actively made. Specifically, there are a printhead configuration in which the density of nozzles of a nozzle array that discharges small ink droplets of the same color is increased to increase the coverage area, a printhead configuration in which an array of nozzles that discharge small ink droplets of the same color and an array of large-diameter nozzles that discharge large ink droplets are provided, and a combination of both.
Among these printhead configurations, there are a printhead in which nozzles in a nozzle array that discharge ink droplets of the same size have different physical shapes and a printhead in which nozzles that discharge ink droplets of different sizes are provided in the same nozzle array. In most of these printheads, as in conventional printheads, the same driving signal is provided for the nozzles of the same array. Misregistration of landing points in a printhead in which nozzles that discharge ink droplets of different sizes are provided in the same array tends to be larger. Therefore, it is becoming difficult to fine-adjust landing points simply by a conventional method of adjusting registration of landing points on a nozzle-array basis.
To solve these problems, there is a technique for adjusting registration of landing points by providing means for inputting multiple driving signals into printing elements in the same nozzle array.
However, the number of nozzles and varieties of ejectable droplet sizes required of printheads are increasing year after year whereas competition to keep prices of inkjet printers down is intensifying. While it is possible to introduce the technique described above to relatively expensive printers, introduction of the technique to low-cost printers is difficult because the technique requires an increased number of driving signal lines to printheads and as many drive timing circuits as the number of printheads, which increase the complexity and costs of the system. Accordingly, most printers integrate signals other than a signal that transmits print data into a common signal (signal line) or integrate signals for discharging ink droplets of the same size into a common signal line.
It is imaginable that, as the number of nozzles of a printhead and the number of varieties of dot sizes increase, demand to integrate signals for driving printheads into a common signal in expensive printers will grow as well. There has been proposed no method for adjusting landing points of nozzles in the same array having different discharge characteristics that is adequate in terms of both cost and performance.